Alternator control system



March 23, 1943. F. JERVIS ALTERNATOR CONTROL SYSTEM Original Filed Sept.16, 1938 2 Sheets-Sheet 1 INVENTOR. FRED JERV/S ATTORNEY.

March 23, 1943' JERVIS I 2,314,501

ALTERNATOR CONTROL SYSTEM Original Filed Sept. 16, 1938 2 Sheets-Shet 2SW/TO CLOSED WHEN O/L FWESSURE /5 LOW g E 5 gig INVENTOR. a & L FREDJERl/IS ATTORNEY.

Patented Mar. 23, 1943 ALTERNATOR CONTROL SYSTEM Fred Jervis, LosAngeles, Calif., assignor, by mesne assignments, to Bardco Manufacturing& Sales Company, Dayton, Ohio, a corporation of California Originalapplication September 16, 1938, Serial N 0. 230,262. Divided and thisapplication May 3, 1941, Serial No. 391,741

8 Claims.

The invention relates to control systems for electric generating plants,and particularly to plants including an alternating current gen- .eratordriven by an internal combustion engine, 7

and is a division of my co-pending application Serial No. 230,262, filedSeptember 16, 1938, now Patent No. 2,255,743, and entitled Control system for electric generating plants.

vAn object of my invention is to provide, in

an electric generating plant including an internal combustion engine asprime mover, means for manually starting the internal combustion safetyand alarm means for disconnecting the generator from the line, stoppingthe prime mover, and energizing an alarm signal when'any of variousdangerous conditions arise in the plant, such as excessive speed,excessive cooling water temperature, low oil pressure, etc.

A further object is to provide means for making inoperative the startingmeans when voltage on the generator reaches a predetermined value andfor maintaining the starting means in inoperative condition so long asthe load on the line to which the generator is connected is such as tonormally cause automatic starting or the generator. 'Still anotherobject is to provide a practical, dependable, and fool-proof control foran electric generating plant in all of its important functions.

These and other apparent objects I attain in 'amanner which will beclear from consideration of the following description taken inconnection with the accompanying drawings, of which:

Fig. 1 shows an electric generating plant and various elements of mycontrol system installed thereon.

Fig. 2 is a diagrammatic illustration of the electric circuit of mycontrol system shown in connection with B-phase alternating currentgenerators.

My control system is illustrated in connection with B-phase alternatingcurrent generators and an internal combustion engine, but it will beunderstood that many of the features of the invention are adaptable toother types of electric generator and other sources of motive power.

In the drawings, numeral 10 designates a 3- phase alternator havingarmature windings A, B, and 0 adapted to have induced thereinalternating electromotive forces having phase relations represented bythe angular positions of the windings. The alternator field II issupplied with direct current from a suitable source such as exciter 12provided with a field 13. The alternator I0 is shown mechanicallycoupled to an internal combustion engine I4 on one side and the exciterl2 on the other. An electric starting motor 15 is provided, adapted toengage a moving part of the engine for rotating it to effect starting.An oil pump it is adapted to be actuated by the engine and to forcelubricating oil under pressure to the bearings of the engine. Numeral I!designates the carburetor or other fuel control device and is operatedby movement of throttle arm 18 to increase or decrease the supply offuel to the engine. Radiator i9 is provided in connection with theengine water jackets for cooling the engine and fan 20, driven by theengine is adapted to force air through the radiator. The whole assemblyis preferably mounted on the frame 2|.

The three windings A, B, and C of alternator H) are connected to oneside of an electromagnetic switch 25 adapted to be normally open and tobe closed by energization of coil 26. The opposite terminals of switch25 are connected to the bus bars I, II, and III in such manner thatclosing of the switch connects windings A, B, and C respectively, tobuses LII, and III. I

A feature of my invention is the provision of means for controlling thefrequency of the E. M. F; generated by the alternator. Two electriccircuit elements differing in their reactive character, such as aninductance coil 21 and a resistance 28, are connected in parallelrelation across one winding of the alternator Hi, there being anactuating coil 29 of relay 30 in series with inductance 21 and anotheractuating coil 3| of the same relay in series with resistance 28, thesetwo parallel circuits thus forming circuits of different power factor.The relay ,30 is a double pole, double throw relay, normally held inopen position, but adapted to be closed toward the right in Fig. 2 byelectromagnetic energization of coil 29 and to be closed toward the leftby electromagnetic energization of coil 3|. When the currents throughcoils 29 and 3| are equal, there is no tendency for the switch arms tobe moved from open position, the magnetic eifects produced by thecurrents in the two coils being equal and opposed; but when the currentthrough either of these actuating coils exceeds that through the other,its magnetic effects on the switch predominates, and the switch isclosed in the corresponding direction. A small motor 32 having a field33 is preferably connected to the relay switch 39 and to the alternatingcurrent supply as shownin such manner that the relay switch 39 acts as areversing switch to reverse the vfield..33 withrespect to the armatureof motor 32 to cause the direction of rotation to be reversed. The motor32 is operatively connected to the throttle arm H) of the carburetor llof the engine I4 as by the worm 34, rack 35, and link 35 to move thethrottle arm upon rotation of the motor 32. The connections are suchthat closing of the relay 30 to the left position in Fig, Zresults inrotation of the motor 32 in sucha direction as to decrease the fuelsupply, to the engine.

The operation of the frequency control means may now be understood.Since the current through an inductive'reactance such as coil 2!decreases as the frequency of the applied voltage increases, and thecurrent through .a resistance such as'resistance. 28 remains.substantially constant as the frequency changes, it will be clear thatif the frequency rises,the current through coil zeor relay 238 will bedecreased while the current through coil 3| will remain nearly the same.The relay will then close to the left position, causing the motor 32 torotate in such direction as to decrease the fuel supplyand reduce thespeed of engine l4. .The frequency will drop correspondingly until thecurrents throughcoilsZQ arid3l of the relay 3!] are again balanced topermit the relay to open. Should the frequency drop below thisequilibrium value. the currentthrough inductance 21 and relay coil 29will increase while the current through resistanoe 28 and coil.3l willremain nearly constant. 'The relay 30 will, therefore, close to theright and cause the motor 32 to rotate in such direction as to increasethe fuel supplied to the engine, whereupon the engine will increase inspeed and cause the frequency to rise again to its equilibrium value.The resistance 28 is preferably variable,and by changing its value, theequilibrium frequency at which the relaySll is in balance maybe altered.For best results, the reactance of coil 3| .is preferably small comparedwith resistance'28. It will be understood that. combinations of circuitelements otherthan an inductance and-aresistance may be employed inplace of -inductance 21 and resistancelfi as, for example, acondenserand a resistance, or a condenser and .aninductance, theessentialrequirement being that the two elements employed diiferin their.relative change in imped ance resulting from change in frequency so asto provide two parallel circuits of different power factor.

.The voltage of the alternator .is maintained substantially constant bysuitable regulating means, regardless of the load. I prefer to employfor this purpose a voltageregulator which is the invention of .OliverA.-Perry, and is the suhiect of his Patent No. 2,236,880, issued April1941. I do notclaim this regulator, per'se, as my invention. In thisregulator, current transformers'40, '41, and'42 are connected in thethree lines leading to bus bars'IfII, 'III respectively. The secondariesof these transformers are connected, respectively, to the inputterminals of the full-wave rectifiers 43, 44, and 45, which arepreferably of the dry oxide type. The output terminals of the rectifiersare connected in series, as shown. The shunt field l3 of exciter I2 isin series with the fixed resistor and the variable resistance 41, thelatter being employed to vary the no-load voltage of the exciter. Thethree rectifiers are connected in series with variable resistance 48 andacross resister 46 in such manner that the output from the rectifiers isapplied across resistor through resistance 43. .The adjustment of theresistance 48 thus controls the potential difference across resistor 46due to current from the rectifiers. The positive terminal of the seriesof rectifiers is connected to that side of the exciter fieldcirouit'tha-t'leads to the negative terminal of the exciter in suchmanner that potential difference due to the rectifier bank adds to thatof the exciter armature to increase the exciter field current.

In the operation of the voltage regulator, the variable resistance 41 isset to secure the desired no load voltage on the atlernator, and as theloadbuilds up in any one or all phases of the alternator, potentialdifferences will, by reason of transformers 40, 4|, and 42, be appliedto the rectifiers 43, 44, and 45. These rectifiers will rectify these A.C. potential differences, and the rectified D. C. potential differenceswill be appliedthrough resistance 48 to the exciter field circuit. Whenthe polarities are as described above, this potential difference due tothe rectifier bank tends to increase the current through eXciter field13, which raises the exciter voltage. which in turn raises thealternator voltage to compensate for the drop in voltage normally causedby an increase in load on the alternator. The resistance 48 is adjustedto provide just the proper amount of compensation required by theparticular alternator l0 employed and the power factor of the load to beencountered. If desired, the resistance 48 may be set forover-compensation to correct forline drop. When properly adjusted, then,the line voltage at a selected point will be held substantially constantfor any load on the alternator; and since the rectifiers 42-, 44, and 45are connected in series, the voltage compensation will be unaffected byan unbalance of the load between phases.

The electric starting motor 15 for the engine 14 is connected incircuitwith a suitable source of electric current, such as a storagebattery 50, through an electromagnetic switch 5| which is normally openbut which is closed by energization of coil 52. One terminal of thebattery 50 is preferably grounded to the frame 2 I. One side of coil 52is connected to the ungrounded terminal of battery 50, and the otherside leads by conductor 53 to one terminal of a push button switch 54,the other side of which is grounded. For manual starting of the enginethe switch 54 is closed which completes the circuit through the coil 52,whereupon the switch 5! closes sending current through the motor l5.

My generating plant may be employed in standby service to take a shareof the load when the current through buses I, II, and III, which maynormally be supplied from another source, such as alternator 55,increases to more than a predetermined value. I, therefore, preferablyprovide .means for automatically starting the engine l4, synchronizingalternator ID with the line, and connecting it therewith. Installedprefand the other pole adapted to connect conductors 62 and 63.Conductor 68 is connected to conductor 53. Conductor 6| leads to oneterminal of the relay switch 64, which in the normal upper positionconnects conductor 62 with the grounded latch release coil 65 of relay66; and in the lower position, occupied when coil 61 is energized,connects conductor 6| through conductor III to one terminal of theswitch of relay 66. The switch of relay 66 is normally held closed andis opened when coil 68 is energized. The coil 68 is connected across onephase of the alternator I8. The relay 86 has a mechanical latch 69adapted to hold open the switch until the latch is lifted byenergization of coil 65. 68 when closed connects conductors I8 and II,the latter connecting with conductor 12 to which are connected oneterminal of each of safety switches 13, 14, and I5.

Switches I3 and are indicated as temperature responsive switches adaptedto close and connect conductor 12 to ground when they reach a certainelevated temperature. Such switches are well-known in the art. SwitchesI3 and I5 are preferably located in parts of the apparatus where it isdesired that the temperature shall always remain less than a certainselected Value. I preferably locate switches 13 and I5 in the head ofthe engine I4 and adjust the switches to close when the temperature ofthe head approaches the boiling temperature of the circulating coolingfluid within. The switch I4 is indicated as a pressure responsive switchadapted to connect conductor 12 to ground when the pressure falls belowa certain selected value. Such switches are well-known in the art. Theswitch I4 is'preferably connected to the discharge passage of the oilpump I 6 of the engine. Since when the engine is running and the pump IBis operating normally, pressure is applied to switch I4, it will beopen; but when the engine is stationary or the pump is not supplying oilunder pressure to the bearings, the switch I4 will close and connectconductor 12 to ground.

t Conductor 62 leads to one terminal of single pole, double throw switch76 which when closed .to the right connects conductor 77 with conductor62, and when closed to the left connects con ductors TI and 63.Conductor 1! is connected to the ungrounded side of the battery.Conductor 68 is connected to one terminal of the coil I8 of relay 79,the other terminal of which is connected to conductor 72. An alarmsignal 88, which may, forexample, be a red lamp or a sound-producingdevice such as a bell, is connected across the coil 78. The relay I9 isnormally in the upper position, connecting conductor 63 with conductor8|, and is moved by energizetion of coil 18 to the lower position inwhich conductor BI is connected through conductor 82 to the ungroundedterminal of starting motor I5. The conductor 8I leads to relay switch 83which is normally held closed, connecting conductor iii to conductor 34.The relay switch 83 is opened by energization of coil 85 which issupplied The switch of relay,

through rheostat 86 by conductors 81 connected to the terminals of theexciter I2. The ignition coil 88 of engine I4, the coil 89 ofelectromagnetic fuel valve 98, and actuating coil 9| of relay switch 92are connected between conductor 84 and ground. The fuel valve 95 ispreferably inserted in the fuel line to the engine and is normallyclosed, but opened when coil 89 is energized.

The relay switch 92 is normally held open but is closed by energizationof coil 9| to connect conductors 93 and 94. Conductor 93 leads to oneline of the alternator I8, for example that connected to winding B,while conductor 94 connects to one terminal of coil 28. The otherterminal of coil 26 is connected through conductor 95 to relay switch 98which, when in its normal closed position, connects conductor 95 withconductor 91. The relay switch 96 is opened when coil 98 is energized.The coil 98 is connected between conductors 83 and H. Conductor 97 isconnected by relay switch 99, when in its normal closed position, withconductor I88. The relay switch 89 is opened by energization of coil I8Iwhich is preferably connected between conductor I88 and conductor I82.The conductor I88 is connected to one terminal of relay switch I83 whichin its closed position connects conductor I88 with a line of alternatorI 8 different from that to which conductor 93 is connected, for example,the line connected to winding C. The relay switch I83 is normally heldopen and is closed when coil I84 is energized to a suitable degree. Thecoil I84 is connected between two of the outgoing power lines fromalternator I8, for example, those connected to windings B and c.

I have illustrated my invention as applied to the case in which thebuses I; II, and III are supplied by alternator which is provided with afield H 8, and an exciter III having a field H2. The windings A, B, andC of alternator 55 may be connected to buses I, II, and III,respectively, through electromagnetic switch II3 which is closed byenergization of coil H4. The connections of the two alternators I8 and55 to the buses I, II, and III are such that the phase rotation of eachalternator is the same relative to the buses to which it is connected,as shown. The neutrals of the windings of alternators I8 and 55 may beconnected through conductor I85. In my installations, alternators I8 and55 may be, and usually are, identical, the alternator 55 being equippedwith voltage regulating means, frequency regulating means, startingmeans, safety and alarmmeans similar to that above described inconnetcion with alternator I8. The connection of r these various deviceswill be obvious in view of the foregoing description. Inasmuch as eitheralternator may inpractice be placed on the line first, and the other beautomatically started and placed on the line when its services arerequired, the series transformer 56 may serve to connect the startingapparatus for either one or the other of the alternators. If switch 51is thrown to the left in Fig. 2, the starting apparatus of alternator I8will be connected; While if switch 5'! is thrown to the right, thestarting apparatus of alternator 55 will be connected through conductorsH5.

For purposes of synchronizing the two alternators, the actuating coilI8I of relay 89 is connected through conductor I82 to the switch ofrelay I IS, the other terminal of which is connected to the winding of.alternator 55 (in this case winding C) to which is adapted to beconnected the winding C of alternator ID to which the relay switch IE3is connected. The relay I I6 is normally held open but is closed byenergization of coil III to a suitable degree. The coil I I! isconnected between windings B and C of alternator 55.

In describing the operation of the apparatus of my invention, I shallassume the alternator 55 connected through switch H3 with the buses I,II, and III and supplying electrical energy to the buses at asubstantially constant voltage. If the engine I5; is to be startedmanually, the switch 16 is closed to its left position, in which theignition coil 88, fuel valve electromagnet 89, and relay coil 9! areenergized through a circuit including relay switch 83, conductor 8|,relay switch Iii in its upper unoperated position, conductor (53, switchI5, and conductor TI. The push button switch 53 is then operated toconnect the starting motor as previously described. If it is desired tohave the alternator I started and connected. automatically to take itsshare of the load, the switch 57 is closed to the left in Fig. 2, theswitch I8 is thrown to the right position connecting conductors TI and62', and the operation is as follows:

As the load current in bus I is increased, a condition will be reached,depending upon the adjustment of relay 59, at which the induced E. M; F.in the winding 53 is sufiicient to energize coils 58 and 6? and causeoperation of relays 59 and 64. Current will then flow from theungrounded side of battery 50 through coil 52, conductor 53, conductor66, relay 59, conductor GI, relay S 1, and conductor 10 to the switch ofrelay 55. Since neither coil 85 nor 58 are energized, the switch ofrelay 66 will be closed. The current will, therefora'pass from conductorI0 through relay 65, to conductor II, and on to conductor I2. Since theengine I4 is stationary, there is no oil pressure and switch I4 isclosed. The current, therefore, passes through switch I4 to ground,completing the circuit. The current passing in this manner through coil52 closes relay 5? and permits current from the battery 50 to passthrough the starting motor I5 to ground, and the starting motor turnsthe engine which presently starts. While this is occurring, ourrent alsopasses from the ungrounded side of the battery through conductor 11,switch 16, relay 5o, conductor coil 18, conductor 12, and oil pressureswitch E4 to ground. The current passing through coil I3 moves theswitch of relay I9 to its lower position. Current then passes throughrelay 5!, conductor 82, relay I9, conductor relay 83, which is nowclosed, to the ignition coil 83, fuel valve 89, and coil 9|, and througheach of these elements back to ground. In this manner the ignitioncircuit of the engine It is energized, and the fuel valve is openedpermitting fuel to flow to the engine. Current passing through coil 9!closes relay 92.

When the engine starts, oil pressure quickly builds up, opening switchI4. The circuit through coil 52 is thus interrupted, opening relay 5!,and disconnecting the starting motor I5. At the same time, the circuitthrough coil I8 is interrupted, permitting the switch of relay 19 to bereturned to its upper position. In this position, current comes fromconductor 63, passes through relay I9, to conductor 8|, and on toenergize the ignition circuit, fuel valve, and coil III as before.

vAs thespeed of theengine I4 increases, the voltage of the alternatorIII also increases, the

with winding B. Current,

voltage regulating means and the frequency regulating means coming intooperation to produce a predetermined voltage and frequency. The relayI03 is adjusted to close at a voltage just below that for which thefield excitation and voltage regulator are set. Accordingly, relay I03closes when the voltage on alternator IEI comes up to normal.

When the alternator I3 is to be started and connected to a dead line,relay 99 is found to be normally closed and when relay I33 closes,current passes from the terminal of the alternator Ii] connecting withwinding C through relay I03, relay 99, conductor 91, relay 96, which isnormally closed, conductor 95, coil 26, conductor 94, relay 92, to theterminal of the alternator connected therefore, flows through coil 25and closes switch 25 connecting alternator III with the line.

If, however, the alternator 55 is in operation supplying the line towhich alternator I0 is to be connected, as I have assumed, it isnecessary to synchronize alternator III with the line supplied byalternator 55 before connecting it thereto, and my apparatus is adaptedto perform this function automatically. The normal frequency and normalvoltage of alternator III are adjusted to be substantially equal tothose of the power circuit to which it is to be connected andthefrequency and voltage regulating means previously described thenmaintain the conditions substantially constant. The relay H5 is set toclose at a voltage just below normal voltage of the alternator 55, so itwill be closed at the time the alternator III is to be synchronized withthe line. The coil IEII of relay 39 will, therefore, be connectedthrough relays I I3 and H6 between the terminal of alternator II]connected to winding C and the terminal of alternator 55 connected towinding C Generally the potential with respect to neutral of thewindings C and C will be different resulting in the passage of currentthrough coil IOI to hold relay 99 open. The relay 99 is set to closeonly when the current through coil 93 falls to a very low value. As thevoltage of alternator I0, then, builds up and the relay I83 closes, theswitch 26 is not closed immediately, but only when the potential ofwinding C becomes nearly equal to the potential of winding C. When thisoccurs, the relay 99 will close, completing the circuit through coil 26,and the switch 25 will close to connect alternator ID to the buses I,II, and III. It will'be observed that since the alternators I0 and 55have been connected to switches 25 and H3 with the same phase rotation,and since the voltages of the two alternators are the same, and thefrequencies are substantially the same, the alternators must be insynchronism when the potential difference between windings C and C isnearly zero, and under these conditions, it is safe to connectalternator III to the power circuit.

When the voltage of alternator I0 comes up to normal, coil 88 isenergized, opening the relay 66. The latch 69 then holds the relayswitch open until released by energization of coil 65. If during theoperation of the alternator its speed should for any reason becomeexcessively high, as might occur due to failure to the frequencyregulating means, the voltage across the tenninals of the exciter I2will rise and cause relay 83, which is set to open at a voltage somewhathigher than normal exciter voltage, to open and break the circuit to theignition coil 88, the fuel valve 89 and the coil 9|. Theengine I 4 willthenstop and since relay c2 will be opened, the current through holdingcoil 26 will be interrupted, resulting in opening of switch 25 anddisconnecting of the alternator ill from the line.

While alternator I is producing normal voltage, if the engine should forany reason become excessively hot, one of the switches 13 and 15 willclose; or should the oil pressure drop to a dangerously low value, theswitch 74 will close. Either of these events causes current to flow fromconductor 63 through coil 18, conductor 12, and the closed switch I3,14, or 15, to ground. Current passing through coil 18 pulls the switchof relay 19 to its lower position breaking the connection betweenconductors 63 and Si through which the ignition coil 88, fuel valve coil89, and relay coil 9| are energized. It will be observed that now whenthe switch of relay 1%) is in its lower position, there is no connectionthrough conductor 82 to the battery, as there was during starting, fornow relay is open. Hence, if either a condition of excessive enginetemperature or one of low oil pressure arises, the engine will be shutdown and the alternator will be disconnected from the line. At the sametime, the alarm signal so will be energized by reason of potentialdifference existing across coil 78 and will indicate to an attendantthat something is wrong. As additional or alternative safety means foropening the switch 25 when one of switches l3, 14, or is grounded, I mayemploy the relay 96, and it will be apparent that current can flow fromconductor 63 through coil 98, and conductor 12 to the grounded switch.This will result in relay 96 opening to break the circuit through theholding coilof switch 25 and the switch 25 will then open.

Should the generator be shut down in any of the ways above described byreason of a dangerous condition existing, it will be impossible for thestarting means to come into operation to again place the generator onthe line unless the load on the line drops to permit relay switches 59and 64 to open. For with the switch of relay 66 held open by latch 69,it is impossible for the starting relay coil 52 to be energized aspreviously explained through the circuit including conductor l0, relay66, conductor TI, and conductor 12. This prevents alternate stopping andstarting of the generator when something is wrong and before anattendant can come to investigate the cause of the alarm signal. If,however, the switch 51 is opened or the current in the line drops tosuch a, low value as to disconnect the starting means, i. e. if the E.M. F. induced in transformer 55 is insufficient to hold relay switch 59closed and relay switch 64 in its lower position, the relay switch 64will be thrown to it upper position completing a circuit from conductor63 through coil 65 to ground; and the current passing through coil 65then releases the latch 69, and permits the switch of relay 66 to close,placing the entire system again in condition for starting of the engine.

It will be seen that I have provided a control system for an electricgenerating plant which is automatic in performing the necessaryfunctions required in operation of a generating plant, and one whichnecessitates a minimum of attention on the part of an attendant, yet oneproviding a high degree of safety.

It will be understood that the embodiment of my invention disclosedherein is illustrative only, and that various changes and modificationsin my system may be made by those skilled in the art without departingfrom the spirit and scope of the appended claims.

I claim as my invention:

1. In an alternating current generating plant including an alternatingcurrent generator, and a prime mover for driving said generator, thecombination of two electric circuits of different power factor connectedin parallel and energized by said generator; and means responsive to thedifferential of currents flowing in said branch circuits for varying thespeed of said prime mover to maintain the frequency of said generatorsubstantially constant 2. In an alternating current generating plantincluding an alternating current generator and a prime mover for drivingsaid generator, the combination of two electric circuits of differentpower factor connected in parallel and energized by said generator;means for varying the speed of said prime mover; a reversible electricmotor for,actuating said means and adapted when ro-' tating in onedirection to increase the speed and when rotating in the oppositedirection to decrease the speed of said prime mover; and a reversingrelay adapted in one closedposition to connect said motor for rotationin one direction and when in the other closed position to connect saidmotor for rotation in the opposite direction, said relay having twoactuating coils connected respectively in said two electric circuits andadapted when energized to urge said relay toward opposite closedpositions, the coil which is} adapted to urge said relay toward theclosed position in which speed is increased being connected in the moreinductive of said circuits.

3. In'an alternating current generating plant including an alternatingcurrent generator and an internal combustion engine for driving saidgenerator, the combination of a throttle on said engine; a reversibleelectric motor operatively connected to said throttle for varying theopening of said throttle; a reversible relay connected to said motor tocontrol the direction of rotation of said motor, said relay being inbalance under the action of two actuating coils; an inductive reactanceelement in series with the one of said coils acting to urge said relayin the direction to connect said motor for opening said throttle; and aresistance element in series with the other of said coils, said coilsand their series connected elements forming circuits of diifering powerfactor, and said circuits being connected in parallel and energized bysaid generator.

4. In an alternating current generating plant including an alternatingcurrent generator and a prime mover for driving said generator, thecombination of: throttle means for controlling the speed of said primemover; a reversible motor; means connecting said motor to said throttleso that when said motor is rotated in one direction the speed of saidprime mover is increased, and when rotated in the opposite directionsaid speed is decreased; a selective circuit connected to the output ofsaid generator and having two branches, one of which has an elementtherein whose impedance increases relatively more rapidly than animpedance element in the other branch when the frequency of the outputof said generator is changed; and a double throw relay having operatingcoils acting in opposition to each other, each coil being connected inone branch of said selective circuit whereby said relay is operated tocause said motor to rotate so as to compensate for any substantialchange of frequency.

including an alternating current generator and a prime mover for saidgenerator, the combination of: means for varying the speed of said primemover; a selective circuit connected to the output of said generator andhaving two branches, one of which has an element therein whose impedanceincreases relatively more rapidly than an impedance element in the otherbranch when the output frequency of said generator is increased; and adouble throw relay having operating coils acting in opposition to eachother, each coil being connected in one branch of said selectivecircuit, whereby said relay is actuated to cause said speed controlmeans to compensate for any substantial change of frequency.

6. In an alternating current generating plant including an alternatingcurrent generator and an internal combustion engine for driving saidgenerator, the combination of a throttle on said engine; a reversibleelectric motor operatively connected to said throttle for varying theopening of said throttle; a reversible relay connected to said motor tocontrol the direction of rotation of said motor, said relay being inbalance under theaction of two actuating coils; a capacitive reactanceelement in series with the one of said coils acting to urge said relayin the direction to connect said motor for closing said throttle; and aresistance element in series with the other of said coils, said coilsand their series connected elements forming circuits of differing powerfactor, and said circuits being connected in parallel and energized bysaid generator.

'7. In an alternating current generating plant 5. In an alternatingcurrent generating plant including an alternating current generator andan internal combustion engine for driving ,said 1 generator, thecombination of: a throttle on said engine; a reversible electric motor:operatively connected to said throttlev for varying the opening of saidthrottle; a reversible relay connected to said motor to control thedirection of rotation. of said motor, said relay being in balance underthe action of two actuating coils; an inductive reactance element inseries with-the one ofsaid coils acting to urge said relay in thedirection to connect said motor for openingsaid throttle;- and acapacitance element in serieswith the other of said coils, said coilsand their series connected elements forming circuits of differing powerfactor, and said circuits being connected in parallel and energized bysaid generator.

energized by said generator and having two parallel branches whoseelectrical 'irnpedances vary differentially with change infrequency ofthe generated current flowing insaid circuit, whereby the currentsflowing in said two branches vary dillerentially in response to suchfrequency change, and meansdifferentially responsive to the magneticeffects of said two currents flowing in said two branches for operatingsaid speed control means in a manner to .compensate for any deviation ofgenerated frequency from a predetermined frequency.

FRED J ERVIS.

